As aircraft operators and manufactures demanded more powerful and efficient engines over the years, engine manufacturers responded by developing gas turbine engines that operate at higher temperatures and pressures. As a result, high-pressure turbine vanes and blades in modern gas turbine engines are often exposed to temperatures near or above the melting point of the superalloys from which they are made. To permit high operating temperatures while preserving the integrity and extending the life of high pressure turbine vanes and blades, engine manufacturers often protect them with multi-layer thermal barrier coatings that comprise an oxidation resistant metallic bond coat and a ceramic layer.
Such coatings are frequently applied to gas turbine engine components by first depositing a metallic bond coat on the part to be protected. After a peening step, which may be optional in some cases, and a cleaning step, the ceramic layer is deposited over the bond coat by electron beam physical vapor deposition (EB-PVD) or another physical vapor deposition method. As part of an EB-PVD process, parts to be coated are placed inside a coating chamber operated at a high vacuum. An electron beam gun inside the coating chamber is magnetically focused on ingots of a ceramic material, for example a mixture of zirconium oxide and yttrium oxide, that has a melting point of more than 4500.degree. F. (2480.degree. C.). The electron beam's energy melts and then vaporizes the ceramic. As the parts to be coated rotate above the ceramic ingots, the vaporized ceramic condenses on the parts, forming the ceramic layer.
Through careful control of operating conditions, a specific and very desirable coating structure, a series of closely packed ceramic columns, and an adherent aluminum oxide scale can be obtained. The ceramic material and the EB-PVD coating process used to make the ceramic layer gives the thermal barrier coating superior thermal insulating properties. The durability of this coating is obtained from the ceramic columnar structure and chemical bond provided by the aluminum oxide scale. This unique columnar structure provides a stress tolerant mechanism to compensate for stresses generated from the difference in thermal expansion coefficients between a metallic turbine component and the ceramic top coat. Coatings like this are described in commonly assigned U.S. Pat. Nos. 4,321,310 to Ulion et al., 4,321,311 to Strangman, 4,401,697 to Strangman, 4,405,659 to Strangman, 4,405,660 to Ulion et al., and 4,414,249 to Ulion et al.
Although such coatings have been used successfully in a variety of applications, there is a constant search for ways to improve the durability of these coatings. Therefore, what is needed in the industry are methods for improving the durability of thermal barrier coatings having columnar structures.